A stream of produced fluids containing hydrocarbon products produced from a subterranean reservoir contains several components that must be separated: a stabilized crude oil generally having a vapor pressure of 14.7 psia or less, condensate, liquefied petroleum gas (LPG) and methane. Condensate refers to a light hydrocarbon mixture that is separated from stabilized crude oil. It typically contains pentane, hexane, and can contain small amounts of butane. These more volatile condensates are often shipped separately from stabilized crude oil. LPG refers to propane, butane and mixtures thereof. In addition to these components, other components that are frequently separated are ethane and water. Further, contaminants such as sulfur and other non-carbon and non-hydrogen elements may also be separated out of the crude oil and gases. A significant amount of capital must be spent for facilities to separate hydrocarbon containing produced fluids into these components.
Heavy and/or waxy crude oils are found in many locations around the world, and are attractive for their low cost per barrel and impressive yield of high-value products. However, high pour points and challenging flow properties complicate the process of transporting these crudes from oilfield to refinery. While transportation by pipeline is usually the preferred method, most heavy crudes are extremely time and temperature-sensitive and must be transferred via insulated or heated tankers to prevent solidification. The addition of chemical pour point depressants or “lighter” petroleum products is a common method of improving flow characteristics of crude oils and thereby alleviating this problem.
Pour point is the lowest temperature at which a liquid will not flow, or the temperature at or below which the liquid loses its ability to flow. It is desirable to keep crude oil at least 11° C., and preferably 22° C., above its pour point during transportation and storage. The lowest sea temperatures are typically around 10° C. To avoid having to heat ships, lines, tanks and the like, crudes should have a pour point less than 30° C., depending on the part of the world.
Typically, the amount of methane or “associated gas” produced along with crude oil is insufficient to justify conversion to Liquefied Natural Gas (LNG). However options to handle associated gas are limited. Natural gas often cannot be burned (flared) due to increasingly stringent regulations around greenhouse gas emissions. Also, natural gas often typically is not reinjected into a producing formation as this dilutes the crude oil and leads to a loss in crude oil production. Local uses, such as combustion of the natural gas for facilities uses, are insufficient to consume this gas.
One technology currently used to handle associated gas that cannot be flared, reinjected or used in local markets is to convert the associated gas into synthetic fuels such as diesel, jet fuel, and naphtha by a Fischer-Tropsch process. Conventional Fischer-Tropsch processes make a waxy product that is subsequently converted via hydroconversion into premium quality transportation fuels. Gas conversion to these products via the Fischer-Tropsch process is well known such as is described in U.S. Pat. No. 7,479,216.
The conventional Fischer-Tropsch conversion process is an extremely exothermic and expensive process and when used on associated gas, facilities distinct from those for crude oil must be used to handle the premium Fischer-Tropsch diesel, jet fuel, condensate and other products. For oilfields with high gas/oil ratio (GOR), gas-to-liquids (GTL) conversion via Fischer Tropsch synthesis is economically unfavorable and may require more space and resources than are available. Likewise, the wax from the Fischer-Tropsch process has such a high melting point that the conventional Fischer-Tropsch product cannot be shipped in conventional crude tankers, but instead, requires expensive ships suitable for handling this high melting temperature material. As described in U.S. Pat. Appln. No. 2006/0069296, conventional crude tankers are often limited to material having pour points at or below 140° F. (60° C.).
Blending the wax from the Fischer-Tropsch process into crude oil is not an option either. Blending as little as 2 wt % of a conventional Fischer-Tropsch product into some crude oils may increase the pour point above 60° C. Also, conventional Fischer-Tropsch products contain substantial quantities of olefins, alcohols and acids. When blended with crude oil these Fischer-Tropsch products can cause the crude oil to be difficult to refine and may lead to a discount in the crude sale price.
There is a need for a low-cost process to convert associated gas from a stream of produced fluids produced from a subterranean formation into a low-impurity synthetic crude oil while avoiding the difficulties caused by wax content. There is a further need for such a process to convert associated gas to a synthetic crude oil that can be blended with natural crude oil, wherein a blended stabilized crude oil having a pour point at or below 30° C. is produced.